1. Field of the Invention
The present invention relates to a rate of penetration sensor for oil drilling rigs and, more particularly, to a method and apparatus for obtaining travelling block position change data on a drilling rig without the use of a measuring line.
2. Description of the Prior Art
In an oil well drilling rig, a travelling block is suspended from a crown block and a drilling apparatus coupled to the travelling block extends into a drilling hole. As drilling proceeds, the drilling apparatus penetrates the earth's surface at a rate which is determined by a number of variables. It is desired to monitor and record the rate of penetration of the drilling apparatus, as well as other events.
The most common method of obtaining the desired rate of penetration information in a drilling rig is to utilize a measuring line directly connected to the travelling block. These devices have problems with respect to reliability because the lines often break. There is also difficulty in making necessary repairs.
As an alternative to direct measurement by means of a wire line, travelling block position may be obtained by determining the amount of cable paid out or wound onto a rotating drum from an arbitrary reference point. This amount of cable is mathematically related to the position of the travelling block in the oil derrick above the rig floor. This relation is a function of the number of feet of cable paid out (or wound in) and the number of lines strung between the crown block and travelling block. A system employing measurment of drum rotation to determine travelling block position is shown in U.S. Pat. Nos. 4,114,435 and 4,156,467 to Patton et al.
Because different layers of cable wound around the drum have different diameters, the amount of cable paid out during each revolution of the drum will not be constant. In order to increase the accuracy of the block position determination, some compensation must be made for varying diameters of wraps about the drum. Patton et al use a computer to accomplish this function. Other measurement devices which utilize electronics to compensate for varying diameters of material wound about a drum are shown in U.S. Pat. No. 3,710,084 to Slagley et al and U.S. Pat. No. 4,024,645 to Giles. Although such systems may provide accurate measurements, the use of an electronic measurement system is not desirable in an oil rig environment because there is frequently no readily available and reliable source of electricity. Even when electricity is available, it may be dangerous to use electrically activated devices due to the possibility of an explosion.
The use of pneumatic devices to measure movement of the travelling block is shown in U.S. Pat. No. 3,750,480 to Dower and U.S. Pat. No. 3,214,762 to Van Winkel. The use of pneumatic devices avoids the problems associated with electronic measurement systems. However, the Dower and Van Winkel systems do not incorporate any type of compensation scheme to assure that accurate measurements are provided despite the varying diameters of cable wraps around the drum. Furthermore, all known systems are relatively complex and cumbersome.